1. Field of the Invention
The present invention concerns a process for coating a substrate by extruding a polyethylene composition thereon. In addition, the present invention concerns an extrusion coating structure.
2. Description of Related Art
Low density polyethylene (PE-LD) made by a high pressure process (referred to as “high pressure PE-LD”) has been used conventionally for extrusion coating. High pressure PE-LD is easy to process, provides adequate moisture barrier and has good sealing properties. The mechanical properties of high pressure PE-LD are, however, not as good as those of other PE grades. This means that a thicker layer of high pressure PE-LD is needed to achieve the mechanical properties required for a coating.
Linear low density polyethylene (PE-LLD), medium density polyethylene (PE-MD) or high density polyethylene (PE-HD) materials exhibit better mechanical properties. On the other hand, a PE composition with a higher density is more difficult to extrude, and the neck-in in extrusion is typically increased as the density increases.
The problem of the combination of good mechanical properties and good processability has been tried to overcome by blending high pressure PE-LD with other PE grades. Another solution has been to coextrude HD, MD or LLD polyethylene together with PE-LD.
On the whole, it is desirable to try to decrease the total amount of polyethylene in a coated product due to the demands of environmental regulations. In particular, it is of importance to decrease the amount of high pressure PE-LD in a coated product, because the production of high pressure PE-LD in the world is decreasing. The present focus in polyethylene production is in the low pressure processes, and thus most often the high pressure PE-LD has to be bought and brought to the plant. This increases the costs of the manufacturer of the extrusion coating.
WO 98/30628 discloses an extrusion coating structure comprising a bimodal ethylene polymer. The ethylene polymer is a blend of at least two different ethylene polymers and it contains 80–100% ethylene repeating units and 20–0% alpha-olefin repeating units. The density of the ethylene polymer is 0.920–0.960 kg/m3.
The ethylene polymer blend can be made in a reactor sequence using a single site or a Ziegler-Natta type catalyst. The catalyst used in the examples is, however, not defined. It is to be noted that in the examples the materials are either blended with 15% of high pressure PE-LD or coextruded with PE-LD. According to our experiments this would indicate that the material in the examples is produced using Ziegler-Natta catalysts. Nothing is mentioned of the advantages of using a single site catalyst, especially of the possibility of extrusion of the composition without blending or coextruding with high pressure PE-LD when using a polyethylene composition produced by single site catalysts.
WO 96/16119 discloses a polyethylene extrusion composition comprising from 75 wt-% to 95 wt-% of at least one ethylene/α-olefin interpolymer composition having a density in the range of 850 kg/m3–940 kg/m3 and from 5 to 25 wt-% of high pressure ethylene polymer. The extrusion composition according to the publication has a melt index equal to or greater than 1 g/110 min. It is stated in the publication that the ethylene/α-olefin interpolymer can be prepared in any conventional way, inter al., by polymerizing in a reactor sequence using a homogeneous single site catalyst. The ethylene/α-olefin interpolymers disclosed in the examples are substantially linear ethylene/1-octene copolymer and a homogeneously branched linear ethylene/1-hexene copolymer. None of these materials is bimodal, and the melt flow properties obtained are due to the long-chain branching of the materials. The catalyst most probably used in the examples is called constrained geometry catalyst, but this is not specifically stated. The polymerization conditions are not defined, either.
U.S. Pat. No. 5,674,342 discloses a process for extrusion coating a substrate. The composition used in the coating process consists according to one alternative solely of ethylene polymer having a Dow Rheology Index (DRI) of at least 0.1. The ethylene polymer is, according to the examples a substantially linear polymer, which is not bimodal. According to the publication it is possible to use either single site catalysts or constrained geometry catalysts in the polymerization. It is stated in the publication that the constrained geometry catalysts are preferably used and that preferably a solution polymerization process is employed. In the examples, however, the used catalyst or process conditions are not indicated.
For the sake of completeness it should be mentioned that cast films (i.e. not extrusion coatings) have been made of substantially linear polymers having a bimodal molecular weight distribution which have been produced by a metallocene catalyst in a two-stage solution process (WO 99/09096). The polymers produced in the different stages were reported to have equal densities. Specifically, the catalyst has the ability to produce long chain branches in the polymer. The processibility of the polymer on the film line was reported to be good. However, based on the disclosure it is not possible to ascertain whether the prior polymers would have a good processability on the extrusion coating line, where the line speed ins 100–500 m/min, compared to the film line having a line speed of a few meters per minute.